Hi Fabrice

Thank you for your reply. It works the way I wanted. Wolfgang also replied and this has lead me to an alternative method. See my reply to him in this mailing list.

Best Wishes

Keith

On 28/04/2022 13:45, Fabrice L wrote:
Sorry, I was a little too enthusiastic, here here is the code :

Le 28 avr. 2022 à 08:41, Fabrice L <fabrice.alpha@gmail.com> a écrit :

Hi Keith,

Le 28 avr. 2022 à 06:05, Keith McKay via ntg-context <ntg-context@ntg.nl> a écrit :

Hi,

In the MWE below I define an rgb colour, myColor1, using random numbers for the r, g and b components. I then use it to fill a unitsquare the size of the page, which works as expected. In the loop that follows I then use the defined colour, myColor1, along with white and the variable r to create shades of myColor1 as in the MWE below. However, myColor1 is not recognised in the loop and seems to be replaced by black, although if I use a predefined colour, in this case BurntSienna from the crayola collection, it works as expected.  I have tried a number of ways to get this to work as shown in the code but without success. I would be grateful for any suggestions.

Best Wishes

Keith McKay

%%%%%%%%%%%%%%%%%%%%MWE%%%%%%%%%%%%%
\setuppapersize[A4,portrait]
\usecolors[crayola]
\starttext
\dorecurse{10}{
\startMPpage
StartPage;
\definecolor[name="myColor1", r = uniformdeviate(1), g = uniformdeviate(1), b = uniformdeviate(1) ] ;
fill unitsquare xscaled PaperWidth yscaled PaperHeight withcolor "myColor1";
for i =2 step 2 until 10:
r := i/10;
fill unitsquare xscaled 5cm yscaled .5cm shifted (1cm,i*cm) withcolor(r[\MPcolor{BurntSienna},white]);
fill unitsquare xscaled 5cm yscaled .5cm shifted (7cm,i*cm) withcolor(r[\MPcolor{myColor1}, white]);
fill unitsquare xscaled 5cm yscaled .5cm shifted (13cm,i*cm) withcolor(r[\MPcolor{"myColor1"}, white]);
%fill unitsquare xscaled 5cm yscaled .5cm shifted (13cm,i*cm) withcolor(r[myColor1 , white]); This line causes an error.
endfor;
StopPage;
\stopMPpage
}

\stoptext
%%%%%%%%%%%%%%%%%%%%%%%%%%%end MWE%%%%%%%%%%%

Here is a solution, with works here, defining in another way the color «  myColor »:

\setuppapersize[A4,portrait]
\usecolors[crayola]
\starttext
\dorecurse{2}{
\startMPpage
StartPage;
color myColor ;
myColor := (uniformdeviate(1),uniformdeviate(1),uniformdeviate(1));
fill unitsquare xscaled PaperWidth yscaled PaperHeight withcolor myColor;
for i =2 step 2 until 10:
r := i/10;
fill unitsquare xscaled 5cm yscaled .5cm shifted (1cm,i*cm) withcolor(r[\MPcolor{BurntSienna},white]);
fill unitsquare xscaled 5cm yscaled .5cm shifted (7cm,i*cm) withcolor(r[\MPcolor{myColor1}, white]);
fill unitsquare xscaled 5cm yscaled .5cm shifted (13cm,i*cm) withcolor(r[\MPcolor{"myColor1"}, white]);
%fill unitsquare xscaled 5cm yscaled .5cm shifted (13cm,i*cm) withcolor(r[myColor1 ,  white]); This line causes an error.
endfor;
StopPage;
\stopMPpage
}

\stoptext

I forgot to remove « myColor1 » from the others parts of the code. 
\setuppapersize[A4,portrait]
\usecolors[crayola]
\starttext
\dorecurse{10}{
\startMPpage
StartPage;
color myColor ;
myColor := (uniformdeviate(1),uniformdeviate(1),uniformdeviate(1));
fill unitsquare xscaled PaperWidth yscaled PaperHeight withcolor myColor;
for i =2 step 2 until 10:
r := i/10;
fill unitsquare xscaled 5cm yscaled .5cm shifted (1cm,i*cm) withcolor(r[\MPcolor{BurntSienna},white]);
fill unitsquare xscaled 5cm yscaled .5cm shifted (7cm,i*cm) withcolor(r[myColor, white]);
fill unitsquare xscaled 5cm yscaled .5cm shifted (13cm,i*cm) withcolor(r[myColor, white]);
%fill unitsquare xscaled 5cm yscaled .5cm shifted (13cm,i*cm) withcolor(r[myColor1 ,  white]); This line causes an error.
endfor;
StopPage;
\stopMPpage
}

\stoptext



Fabrice.
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